1 COMET Hydrometeorology 00-1 Matt Kelsch Tuesday, 19 October 1999 Radar-Derived Precipitation Part 1 I.Radar Representation of Precipitation II.WSR-88D, PPS III.PPS Adjustment, Limitations IV.Effective Use

2 II.Radar Representation of Precipitation 1. Sampling Issues Vertical Inconsistency Range Degradation Precip Phase (bright band, hail, snow) Ground Clutter/Terrain Blocking 2. The reflectivity to rainfall rate (Z- R) conversion

3 1.Sampling Issues Radar domain cannot be sampled at consistent elevations, with consistent bin volumes, or for precipitation with similar stage of development or phase. Beam elevation increases with distance--low-level precip not represented well Bin size increases with distance--incomplete beam filling, smearing of small-scale structure Ground clutter and terrain features may block low level information Changes in precip phase have inconsistent effects--bright band, hail contamination Radar Representation of Rainfall

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11 16 sep99: Storm Total Radar-derived Accumulationfrom KRAX (Raleigh NC)

12 16 sep99: Storm Total Radar-derived Accumulationfrom KAKQ (Wakefield VA)

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16 Bright Band 

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18 Radar Representation of Rainfall 2. Deriving rainfall (the Z-R conversion) Power returned to the radar is related to the rainfall intensity, but not in a consistent, easily- modeled way. –Offers a good quantitative approximation –Preserves important qualitative precip structure –Varies with drop size distribution and precip phase

19 Historical Background Marshall, J. S. and W. McKay Palmer, 1948: The distribution of raindrops with size, J. Meteor., 5, Spilhaus, A. F., 1948: Drop size, intensity, and radar echo, J. Meteor., 5, Chapman, G., 1948: Size of raindrops and their striking force at the soil surface in a red pine plantation, Trans. Amer. Geophys. Union, 29, Horton, R. E., 1948: Statistical distribution of drop sizes and the occurrence of dominate drop sizes in rain, Trans. Amer. Geophys. Union, 29, Atlas, D., 1990: Radar in Meteorology, pp Austin, P., 1987: Relation between measured radar reflectivity and surface rainfall, Mon. Wea. Rev., 115,

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27 Z-R Relationships WSR-88D, Marshall-Palmer (general), and Tropical

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